Upstream: added sticky sessions support for upstreams.

Sticky sessions allow to route the same client to the same upstream server.

- upstream structures are extended to keep session-related information

- existing balancing modules are updated to provide an id of the selected
  server (SID) in pc->sid, and to select the server, given it's SID.

- other balancing modules are allowed to set the pc->hint value to choose
  the desired peer.  The sticky module will not change the hint if it's
  already set.

- the feature is enabled by default and can be disabled with the
  "--without-http_upstream_sticky" switch of the configure script.

The following configuration can be used to enable sticky sessions for
supported balancing modules:

    upstream u1 {
        server 127.0.0.1:8080;
        server 127.0.0.1:8081;

        sticky cookie server_id expires=1h domain=.example.com path=/;
    }

Co-authored-by: Ruslan Ermilov <ru@nginx.com>
Co-authored-by: Roman Arutyunyan <arut@nginx.com>
Co-authored-by: Maxim Dounin <mdounin@mdounin.ru>

auto/modules

@@ -950,6 +950,20 @@ if [ $HTTP = YES ]; then
         . auto/module
     fi
 
+    if [ $HTTP_UPSTREAM_STICKY = YES ]; then
+        have=NGX_HTTP_UPSTREAM_STICKY . auto/have
+        have=NGX_HTTP_UPSTREAM_SID . auto/have
+
+        ngx_module_name=ngx_http_upstream_sticky_module
+        ngx_module_incs=
+        ngx_module_deps=
+        ngx_module_srcs=src/http/modules/ngx_http_upstream_sticky_module.c
+        ngx_module_libs=
+        ngx_module_link=$HTTP_UPSTREAM_STICKY
+
+        . auto/module
+    fi
+
     if [ $HTTP_STUB_STATUS = YES ]; then
         have=NGX_STAT_STUB . auto/have